1. Field of the Invention
The present invention relates to a compound semiconductor device.
2. Description of the Related Art
As substitutions for conventional silicon semiconductor devices, the development of nitride-compound semiconductor devices having the potential to operate at higher speed has been advanced. Among such compound semiconductor devices, in particular, GaN semiconductor devices are being actively researched and developed.
Such a GaN semiconductor material has a hexagonal crystal structure. With typical semiconductor devices formed of a hexagonal crystal semiconductor material, the c plane is employed. Such a GaN semiconductor material has two polar planes, i.e., the Ga-plane (Ga-polar) and the N-plane (N-polar). In general, it is difficult to grow such a crystal structure in the N-polar direction. Accordingly, as typical substrates, epitaxial substrates (wafers) are employed, which are obtained by growing such a crystal structure in the Ga-polar direction. FIG. 1A is a cross-sectional view of such a GaN semiconductor device.
A GaN semiconductor device 2r includes an epitaxial substrate 10. The epitaxial substrate 10 includes a growth substrate 12, a GaN layer 14, and an AlGaN layer 16. The GaN layer 14 is configured as a buffer layer and as an electron transport layer. The GaN layer 14 is formed on the growth substrate 12 such as a SiC substrate by means of crystal growth in the Ga-polar direction. Furthermore, the AlGaN layer 16 configured as an electron supply layer is formed on the GaN layer 14 by means of epitaxial growth. Such a GaN semiconductor device has a Ga-plane as a device surface. That is to say, semiconductor elements such as HEMTs (High Electron Mobility Transistors) or the like are formed on the Ga-plane side. The development of such a GaN semiconductor device 2r for practical use is being advanced. Examples of usage thereof include semiconductor devices employed in a wireless communication base station, and the like. In the present specification, a transistor (HEMT) formed in the GaN semiconductor device 2r shown in FIG. 1A will be referred to as the “Ga-plane HEMT”.
In order to provide such a HEMT with a high operation speed, it is important to reduce access resistance. It can be assumed that such access resistance is equivalent to a series connection of a contact resistance component Rc and a semiconductor resistance component. With such a Ga-plane HEMT, a channel 18 is formed in the GaN layer 14. However, the AlGaN layer 16, which is configured as an electron supply layer, acts as a barrier that suppresses contact between the channel 18 and a drain electrode or otherwise a source electrode. This leads to a problem of a large contact resistance Rc.
As a substitution, a GaN Semiconductor device 2 has been proposed having a structure in which semiconductor elements are formed on the N-plane side (Singisetti, Uttam, Man Hoi Wong, and Umesh K. Mishra, “High-performance N-polar GaN enhancement-mode device technology”, Semiconductor Science and Technology 28.7 (2013):074006). FIG. 1B is a cross-sectional view of such a GaN compound semiconductor device. In the present specification, a transistor formed in the GaN semiconductor shown in FIG. 1B will be referred to as the “N-plane HEMT”, which is distinguished from the Ga-plane HEMT shown in FIG. 1A. A GaN semiconductor device 2s includes an epitaxial substrate 20. The epitaxial substrate 20 includes a growth substrate 22, a GaN layer 24, an AlGaN layer 26, and a GaN layer 28. The GaN layer 24 is configured as a buffer layer. The GaN layer 24 is formed on the growth substrate 22 such as a SiC substrate or the like by means of crystal growth in the N-polar direction. Furthermore, the AlGaN layer 26 configured as an electron supply layer is formed on the GaN layer 24 by means of epitaxial growth. Moreover, the GaN layer 28 configured as an electron transport layer is formed on the AlGaN layer 26 by means of epitaxial growth.
With such a GaN semiconductor device 2s, each channel 30 of a given HEMT is formed in the GaN layer 28. Accordingly, there is no AlGaN layer 26 that acts as an energy barrier between the channels 30 and the drain electrode and the source electrode formed on the surface layer side. Such an arrangement allows an ohmic contact to be provided, thereby allowing the contact resistance Rc to be reduced. Furthermore, the AlGaN layer 26 is arranged closer to the growth substrate 22 side than each channel 30. This leads to the formation of a back barrier structure, thereby suppressing the short-channel effect. Based on the reasons described above, in principle, such N-plane HEMTs have improved high-frequency characteristics as compared with Ga-plane HEMTs.
However, it is extremely difficult to provide such crystal growth in the N-polar direction as compared with crystal growth in the Ga-polar direction, as described in the Non-patent document (Zhong, Can-Tao, and Guo-Yi Zhang, “Growth of N-polar GaN on vicinal sapphire substrate by metal organic chemical vapor deposition”, Rare Metals 33.6 (2014), pp709-713). At present, mass-produced N-plane HEMTs are not known. That is to say, such N-plane HEMTs are still at the basic research stage. In addition, manufactured crystal materials have a problem of poor quality. Accordingly, the N-plane HEMTs formed on such a crystal material have poor characteristics, which fall far short of the theoretical expected values.